NEW YORK, NY - JUNE 17: Hemp oil products are displayed during the Cannabis World Congress & Business Expo at the Jacob Javits Center on June 17, 2016 in New York City. (Photo by Justin Sullivan/Getty Images)

The U.S. Food and Drug Administration sent letters on Tuesday to four cannabis companies, warning them against making medical claims about cannabidiol (CBD). The agency also took issue with the businesses marketing CBD products as dietary supplements.

It's certainly not the first time the agency has gone after CBD producers. The FDA sent similar letters to more than a dozen companies in 2015 and 2016. But the latest warnings are notable in that they target one of the biggest players in the CBD market.

CBD is a non-psychoactive compound that is found in both marijuana and hemp plants. The cannabinoid has been gaining mainstream recognition ever since CNN covered the story of Charlotte Figi, a pediatric epilepsy patient in Colorado. Figi was 5-years-old and having 300 seizures a week. She saw a dramatic reduction in seizures after she tried cannabis oil.

Figi's parents went to the Stanley Brothers for help to find a strain of cannabis that was high in CBD and low in THC, the psychoactive cannabinoid. The Stanley Brothers developed a strain they dubbed "Charlotte's Web" after the girl. Since then, Charlotte's Web has gained so much traction that it has become a brand in its own right. The extracts were even briefly featured on Target.com, until the company removed them amid media attention.

There are more than 200 brands of hemp-derived CBD, according to a report from the Brightfield Group. Of those brands, CW Hemp dominates the space with the largest market share. The market is estimated to be worth about $287 million this year and is expected to hit $1 billion by 2020.

Brightfield Group

A look at the top 10 brands in the CBD industry (Image by Brightfield Group)

Some CBD producers seem to be hoping that marketing their products as dietary supplements would help them escape scrutiny from the FDA. CW Hemp's website even has the following disclaimer: "These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease."

But the FDA sees it differently. "The claims on your websites establish that the products are drugs," read the agency's letter to CW Hemp CEO Joel Stanley. "They are intended for use in the diagnosis, cure, mitigation, treatment, or prevention of disease and/or because they are intended to affect the structure or any function of the body."

John-David A. Belfontaine, the CEO of CBD producer Phivida, said that his company is careful to "avoid making direct health claims on the CBD contained in its products." But the company does encourage consumers to search for "cannabidiol" at PubMed to access "thousands of randomized clinical trials on CBD and its effect on range of disorders, from; cancer, to general pain and inflammation."

The agency's previous warning letters do not seem to have slowed down the market for CBD products, which are in high demand in regions where marijuana is difficult to access. Consumers in the Midwest use the product more frequently than any other region, according to the Brightfield report.

Meanwhile, 29 states and D.C. have comprehensive medical marijuana laws on the books, and 17 states have legalized medical CBD oil.

This post has been updated with comment from John-David A. Belfontaine.